The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No. 10-101822, filed Mar. 31, 1998
Japanese Patent Application No. 10-197610, filed Jul. 13, 1998
Japanese Patent Application No. 11-18596, filed Jan. 27, 1999
1. Field of the Invention
The present invention relates to an optical filter and an optical device provided with this optical filter.
2. Description of the Related Art
In a digital still camera employing an imaging device such as a CCD (hereafter a digital still camera is simply referred to as a xe2x80x9cDSCxe2x80x9d in this specification), xe2x80x9cbeatxe2x80x9d interference may occur as a result of a certain relationship between the spatial frequency of the subject image and the repetitive pitch of dot-type on-chip color separation filters provided at the front surface of the imaging device. In order to prevent any false color signals from being generated by the beat, i.e., in order to prevent the so-called xe2x80x9ccolor moire,xe2x80x9d an optical low-pass filter is provided between the taking lens and the imaging device. The optical low-pass filter, which is constituted by employing a birefringent plate achieving birefringence, reduces the generation of the beat through the birefringent effect provided by the birefringent plate. Normally, quartz is employed to constitute the birefringent plate.
Japanese Examined Patent Publication No. 1994-20316 proposes an optical low-pass filter employing two birefringent plates such as that described above, which is suited for application in an imaging device provided with dot-type on-chip color separation filters. This optical low-pass filter is constituted by enclosing a quarter-wave plate between two birefringent plates with the directions in which the image becomes shifted through the birefringence offset by approximately 90xc2x0 from each other.
Now, the so-called direct image forming system, in which the imaging device is directly provided at the primary image forming plane of the taking lens without employing a reduction lens system or the like is becoming the mainstay in single lens reflex type DSCs that allow interchange of the taking lens among DSCs in recent years. The advent of the direct image forming system has been realized through the utilization of imaging devices having a large image area of approximately 15.5 mmxc3x9722.8 mm that have been manufactured in recent years to replace ⅔xe2x80x3 size (approximately 6.8 mmxc3x978.8 mm) and 1xe2x80x3 size (approximately 9.3 mmxc3x9714 mm) imaging devices that have been conventionally used in television cameras and the like. With this size of image area available, an image plane having a size (approximate aspect ratio 2:3=15.6 mmxc3x9722.3 mm), which is comparable to the image plane size of the C-type silver halide film IX240 system (APS), is achieved. By employing an imaging device achieving a relatively large image area, it becomes possible to adopt a camera system that employs the 135-type photographic film in a DSC. To explain this point, providing a ⅔xe2x80x3 size or 1xe2x80x3 size imaging device at the field of a camera using the 135-type film only achieves a small image plane size for the imaging device compared to the image plane size of the 135-type film (24 mmxc3x9736 mm). As a result, a large difference will manifest in the angle of field achieved by a taking lens having a specific focal length, to cause the photographer to feel restricted. This problem becomes eliminated as the image area of the imaging device increases and becomes closer to the image plane size of the 135-type film.
However, as the image area in a single lens reflex type DSC, which forms the primary image with the taking lens at an image device directly, increases, the problems explained below arise to a degree to which they cannot be neglected.
Imaging devices in DSCs in recent years have evolved in two directions, i.e., toward a higher concentration of pixels and toward a larger image plane. When the number of pixels is increased to exceed 1 million pixels while maintaining the size of the image plane at approximately ⅓xe2x80x3 to xc2xdxe2x80x3 as in the prior art, the pixel pitch becomes reduced. For instance, in an imaging device having approximately 1,300,000 pixels, with its image plane size at approximately ⅓,xe2x80x3 the pixel pitch is approximately 4 xcexcm. Generally speaking, the pixel pitch xe2x80x9cpxe2x80x9d at an imaging device and the thickness xe2x80x9ctxe2x80x9d of the birefringent plates constituting the optical low-pass filter which is employed to support the pixel pitch xe2x80x9cpxe2x80x9d achieve the relationship expressed through the following equation (1)
p=t(ne2xe2x88x92no2)/(2nexc3x97no)xe2x80x83xe2x80x83(1)
with
t: birefringent plate thickness
ne: extraordinary ray refractive index at birefringent plate
no: ordinary ray refractive index at birefringent plate
When a quartz plate, which is most commonly employed to constitute a birefringent plate, is used in an imaging device with a pixel pitch of approximately 4 xcexcm, the thickness xe2x80x9ctxe2x80x9d required of the quartz plate is concluded to be approximately 0.7 mm by working backward with xe2x80x9cpxe2x80x9d in equation (1) set at 4 xcexcm, since the refractive indices of quartz for light having a wavelength of 589 nm are ne=1.55336 and no=1.54425. Since the thickness of the quarter-wave plate needs to be approximately 0.5 mm regardless of the pixel pitch p, the entire thickness achieved when constituting an optical low-pass filter by pasting together three plates, i.e., two quartz plates (birefringent plates) and one quarter-wave plate, will be approximately 2 mm.
However, when the area of the photosensitive surface of an imaging device increases, as in the case of, in particular, an imaging device employed in a single lens reflex type DSC, it becomes necessary to increase the thickness of the optical low-pass filter for the reasons detailed below.
While the size of the image plane of a ⅓xe2x80x3 imaging device is approximately 3.6 mmxc3x974.8 mm, let us now consider an imaging device having an image plane size equivalent to that of the C-type (aspect ratio 2:3=16 mmxc3x9724 mm) in an IX240 system (advanced photo system (APS)) with silver halide film. When pixels are arrayed at a pixel pitch of approximately 4 xcexcm on this imaging device, the total number of pixels for the entire image plane will exceed 20 million by simple calculation, and it is considered that the current technical level is not high enough to realize such a large number of pixels for practical use from the viewpoints of the yield in imaging device production, the scale and processing speed of the image information processing circuit and the like. As a result, it is assumed that it is appropriate to set the number of pixels at approximately two million and several hundreds of thousands in an imaging device having a large image plane equivalent to that of the APS-C type, which sets the pixel pitch at 10 and several xcexcm.
For instance, when an APS-C size imaging device (16 mmxc3x9724 mm) is prepared at a pixel pitch set to 12 xcexcm, the number of pixels in the imaging device will be approximately 2,670,000. When constituting the birefringent plates of the optical low-pass filter employed in combination with the imaging device having the pixel pitch of 12 xcexcm with quartz, the thickness of a single quartz plate is calculated to be xe2x80x9ctxe2x80x9d=2.04 mm by incorporating xe2x80x9cpxe2x80x9d=12 xcexcm in equation (1). By adding the thicknesses of two such quartz plates and a quarter-wave plate (0.5 mm), the thickness of the optical low-pass filter is calculated to be 4.58 mm, which is more than twice as large as the thickness of an optical low-pass filter (thickness: 2 mm) with the pixel pitch set at 4 xcexcm.
In addition, since the spectral sensitivity of an imaging device is different from the spectral sensitivity of the human eye, an IR blocking filter is normally provided to cut off infrared light within the imaging optical path in a DSC employing an imaging device. This IR blocking filter (thickness; approximately 0.8 mm) is also provided pasted to the optical low-pass filter. Thus, the entire thickness of the optical low-pass filter supporting the pixel pitch of 12 xcexcm will go up to 5.38 mm when the thickness of the IR blocking filter is included.
It is difficult to place an optical low-pass filter having such a thickness between a taking lens and an imaging device. Even in the case of a regular lens shutter type DSC, which does not require any member to be provided between the rear end of the taking lens and the photosensitive surface of the imaging device except for the optical low-pass filter, it must be ensured in design that the minimum value (the so-called back focal distance) of the distance between the rearmost end of the taking lens and the imaging device is larger than the thickness of the optical low-pass filter. Setting the length of the back focal distance of the taking lens larger than the focal length of the taking lens imposes restrictions in terms of the optical design.
Furthermore, in the case of a single lens reflex type DSC, which directly forms an image of the subject achieved by a taking lens on a large size imaging device without employing a reduction lens system, a quick return mirror for switching the optical path between the viewfinder and the imaging system or a fixed semitransparent mirror (beam splitter) is needed between the taking lens and the imaging device. In addition, a mechanical shutter is required for defining an exposure time and for blocking the imaging device from exposure during an image signal read operation at the imaging device. While this structure having a mirror and a shutter provided between the taking lens and its image forming plane is also adopted in a single lens reflex camera that employs regular silver halide film, it is difficult to provide an optical low-pass filter having a thickness exceeding 5 mm in addition while ensuring that it does not present any obstacle in the operation or the mirror at the shutter. It merits particular note that more and more cameras in recent years adopt the autofocus (AF) function, and that a single lens reflex camera with the AF function adopts a structure having a sub mirror provided to the rear of the quick return mirror, i.e., between the quick return mirror and the shutter, to guide light flux to a focal point detection device. This makes it even more difficult to position an optical low-pass filter having a thickness exceeding 5 mm.
In addition to the problem of an increased thickness of the optical low-pass filter resulting from a larger pixel pitch in a larger imaging device as described above, another problem arises as detailed below.
Normally, the length of the air equivalent optical path achieved when light is transmitted and advances through a medium having a thickness xe2x80x9ctxe2x80x9d and a refractive index xe2x80x9cnxe2x80x9d is expressed as t/n. In other words, the air equivalent optical path lengths achieved when light advances through media having the same refractive index xe2x80x9cnxe2x80x9d but having different thicknesses xe2x80x9ctxe2x80x9d, vary. Now, let us consider light emitted from one point on the optical axis of a photographic optical system toward an imaging device to reach the center of the image plane of the imaging device and light emitted from the same point on the optical axis of the photographic optical system toward the imaging device to reach the periphery of the image plane.
Since the light that reaches the center of the image plane enters the light entry surface of the optical low-pass filter at almost a right angle, xe2x80x9ctxe2x80x9d roughly equals the thickness of the optical low-pass filter. In contrast, since the light reaching the periphery of the image plane advances diagonally through the optical low-pass filter, xe2x80x9ctxe2x80x9d here is larger than the xe2x80x9ctxe2x80x9d encountered by the light reaching the center of the image plane. Since the lengths of the air equivalent optical paths achieved by light being transmitted through the optical low-pass filter are different for the light reaching the center of the image plane and the light reaching the periphery of the image plane as explained above, a focus misalignment occurs in the direction of the optical axis between the image plane center and the image plane periphery. The degree of this focus misalignment increases as the thickness of the optical low-pass filter increases as described above, which may result in a reduced image quality at the peripheral area of the image plane.
As the size of the imaging device is increased, a problem of foreign matter becoming transferred as explained next, i.e., a problem of foreign matter such as dust and lint adhering to the photosensitive surface of the imaging device to cast a shadow onto the image captured by the imaging device, tends to occur readily, in addition to the problems discussed above. In particular, in an interchangeable lens type DSC in which foreign matter such as dust and lint readily enters the mirror box when the taking lens is detached, this problem is more pronounced.
A similar problem occurs in optical devices such as facsimile machines and image scanners when foreign matter such as dust and lint materialize as a document is transmitted or the document read unit moves, which may become adhered to the vicinity of the photosensitive surface of the photoelectric conversion element or the glass (platen glass) upon which the document is placed to result in a shadow being cast on the input image, as in the interchangeable lens type DSC.
Now, since the crystal of quartz employed to constitute birefringent plates imparts a piezoelectric effect, the crystal itself is caused to become electrically charged readily by vibration or the like. The quartz crystal also has a property that does not allow a stored electrical charge to be discharged easily. In addition, since an insulating material such as plastic, ceramic or the like is employed to constitute the imaging device package, the electrical charge stored at the imaging device cannot be released with ease.
Vibration and air currents occurring as a result of an operation of an optical device sometimes cause the foreign matter discussed above to become suspended inside the optical device, which may ultimately become adhered to the electrically charged birefringent plates, imaging device or the like, as explained above. Consequently, the operator of the optical device is required to clean the optical device frequently to prevent shadows from being cast as explained earlier.
A first object of the present invention is to provide an optical filter achieving a small thickness and an optical device provided with the optical filter.
A second object of the present invention is to provide an optical low-pass filter which is capable of preventing the loss of image quality at the periphery of the image plane even when the image area at the imaging device is expanded or even when the pixel pitch is increased, and an optical device provided with the optical low-pass filter.
A third object of the present invention is to prevent foreign matter from becoming adhered to the optical filter described above, a photoelectric conversion element and the like to cast shadows thereupon, by neutralizing an electrical charge occurring as a result of the optical filter, the photoelectric conversion element and the like becoming electrically charged.
In order to achieve the objects described above, the present invention comprises a first birefringent plate constituting an optical element that spatially divides incident light into two separate light fluxes along a first direction extending perpendicular to the direction in which the incident light advances, a vibrational plane converting plate that changes the vibrational planes of the two light fluxes emitted from the first birefringent plate and a second birefringent plate constituting an optical element that spatially divides each of the two light fluxes emitted from the vibrational plane converting plate into two light fluxes along a second direction that is different from the first direction to achieve a total of four separate light fluxes, with at least either the first birefringent plate or the second birefringent plate, constituted of a material having a larger difference between the extraordinary ray refractive index and the ordinary ray refractive index compared to that of quartz.
In addition, according to the present invention, an anti-reflection coating is applied to a boundary surface of the first birefringent plate and an optical element provided adjacent to the first birefringent plate and a boundary surface of the second birefringent plate and an optical element provided adjacent to the second birefringent plate.
Furthermore, according to the present invention, the vibrational plane converting plate is constituted of a phase plate that is capable of creating a phase difference of a specific quantity between a light component that vibrates in one vibrating direction and a light component that vibrates in another vibrating direction extending perpendicular to the one vibrating direction for each of the two light fluxes emitted from the first birefringent plate.
According to the present invention, the vibrational plane converting plate may be constituted of an optical rotatory plate provided as an optical element that rotates the directions of vibration of the two light fluxes emitted from the first birefringent plate at the vibrational plane by a specific degree.
Alternatively, the present invention comprises a first birefringent plate for spatially dividing light emitted from an image forming lens along a first direction to achieve two separate light fluxes, a phase plate that creates a phase difference of a specific quantity between a light component that vibrates in one vibrating direction and a light component that vibrates in another vibrating direction extending perpendicular to the one vibrating direction for each of the two light fluxes emitted from the first birefringent plate and a second birefringent plate having almost the same thickness and almost the same refractive index as those of the first birefringent plate, provided for spatially dividing each of the two light fluxes emitted from the phase plate along a second direction that is different from the first direction to achieve two separate light fluxes to be guided to the imaging plane of the imaging device, with the thickness t1 and the refractive index n1 of the first birefringent plate and the second birefringent plate satisfying the following conditional equation, with A representing the image height at the image plane corners, PO representing the air equivalent optical path length extending from the imaging plane to the exit pupil of the image forming lens and A/POxe2x89xa70.15 satisfied.                     t1        ≦                  C          xc3x97                      n1                          Y              ⁢                              xe2x80x83                            ⁢                              (                n1                )                                                                        (        2        )            
with                               Y          ⁢                      xe2x80x83                    ⁢                      (            n1            )                          =                  1          -                                    cos              ⁢                              xe2x80x83                            ⁢              φ                                      cos              ⁢                              xe2x80x83                            ⁢                              θ                ⁢                1                                                                        (        3        )                                C        =                              1            2                    xc3x97                      {                                          K                xc3x97                B                xc3x97                d                xc3x97                Fno                            -                                                (                                      1                    -                                                                  cos                        ⁢                                                  xe2x80x83                                                ⁢                        φ                                                                    cos                        ⁢                                                  xe2x80x83                                                ⁢                                                  θ                          ⁢                          2                                                                                                      )                                xc3x97                                  t2                  n2                                                      }                                              (        4        )                                          sin          ⁢                      xe2x80x83                    ⁢                      θ            ⁢            1                          =                              sin            ⁢                          xe2x80x83                        ⁢            φ                    n1                                    (        5        )                                          θ          ⁢          1                =                              sin                          -              1                                ⁡                      (                                          sin                ⁢                                  xe2x80x83                                ⁢                φ                            n1                        )                                              (        6        )                                          θ          ⁢          2                =                              sin            1                    ⁡                      (                                          n1                xc3x97                sin                ⁢                                  xe2x80x83                                ⁢                φ                            n2                        )                                              (        7        )                                0.25        ≦        K        ≦        0.35                            (        8        )                                1        ≦        B        ≦        3                            (        9        )            
t1: thicknesses of the first birefringent plate and the second birefringent plate
n1: refractive indices of the first birefringent plate and the second birefringent plate
t2: thickness of the phase plate
n2: refractive index of phase plate
d: pixel pitch at the imaging device
xcfx86: Angle of incidence at a first birefringent plate of light flux entering corner of the imaging plane of the imaging device from the center of the exit pupil of the taking lens
Fno: F number of the taking lens
Alternatively, the present invention may comprise a first birefringent plate for spatially dividing light emitted from an image forming lens along a first direction to achieve two separate light fluxes, a phase plate that creates a phase difference of a specific quantity between a light component that vibrates in one vibrating direction and a light component that vibrates in another vibrating direction extending perpendicular to the one vibrating direction for each of the two light fluxes emitted from the first birefringent plate and a second birefringent plate having almost the same refractive index as that of the first birefringent plate, provided for spatially dividing each of the two light fluxes emitted from the phase plate along a second direction that is different from the first direction to achieve two separate light fluxes to be guided to the imaging plane of the imaging device, with the thickness t11 and the refractive index n1 of the first birefringent plate and the thickness t12 and the refractive index n1 of the second birefringent plate satisfying the following conditional equation, with A representing the image height at the image plane corners, PO representing the air equivalent optical path length extending from the imaging plane to the exit pupil of the image forming lens and A/POxe2x89xa70.15 satisfied.                               t11          +          t12                ≦                  C1          xc3x97                      n1                          Y              ⁢                              xe2x80x83                            ⁢                              (                n1                )                                                                        (        10        )            
with                               Y          ⁢                      xe2x80x83                    ⁢                      (            n1            )                          =                  1          -                                    cos              ⁢                              xe2x80x83                            ⁢              φ                                      cos              ⁢                              xe2x80x83                            ⁢                              θ                ⁢                1                                                                        (        11        )                                C1        =                              K            xc3x97            B            xc3x97            d            xc3x97            Fno                    -                                    (                              1                -                                                      cos                    ⁢                                          xe2x80x83                                        ⁢                    φ                                                        cos                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      ⁢                      2                                                                                  )                        xc3x97                          t2              n2                                                          (        12        )                                          sin          ⁢                      xe2x80x83                    ⁢                      θ            ⁢            1                          =                              sin            ⁢                          xe2x80x83                        ⁢            φ                    n1                                    (        13        )                                          θ          ⁢          1                =                              sin                          -              1                                ⁡                      (                                          sin                ⁢                                  xe2x80x83                                ⁢                φ                                            n                ⁢                1                                      )                                              (        14        )                                          θ          ⁢          2                =                              sin                          -              1                                ⁡                      (                                          n1                xc3x97                sin                ⁢                                  xe2x80x83                                ⁢                φ                            n2                        )                                              (        15        )                                0.25        ≦        K        ≦        0.35                            (        16        )                                1        ≦        B        ≦        3                            (        17        )            
t11: thickness of the first birefringent plate
t12: thickness of the second birefringent plate
n1: refractive indices of the first birefringent plate and the second birefringent plate
t2: thickness of the phase plate
n2: refractive index of phase plate
d: pixel pitch at the imaging device
xcfx86: Angle of incidence at a first birefringent plate of light flux entering corner of the imaging plane of the imaging device from the center of the exit pupil of the taking lens
Fno: F number of the taking lens
Alternatively, the present invention may comprise a first birefringent plate for spatially dividing light emitted from an image forming lens along a first direction to achieve two separate light fluxes, a phase plate that creates a phase difference of a specific quantity between a light component that vibrates in one vibrating direction and a light component that vibrates in another vibrating direction extending perpendicular to the one vibrating direction for each of the two light fluxes emitted from the first birefringent plate and a second birefringent plate having a different thickness and a different refractive index from those of the first birefringent plate, provided for spatially dividing each of the two light fluxes emitted from the phase plate along a second direction that is different from the first direction to achieve two separate light fluxes to be guided to the imaging plane of the imaging device, with the thickness t11 and the refractive index n11 of the first birefringent plate and the thickness t12 and the refractive index n12 of the second birefringent plate satisfying the following conditional equation, with A representing the image height at the image plane corners, PO representing the air equivalent optical path length extending from the imaging plane to the exit pupil of the image forming lens and A/POxe2x89xa70.15 satisfied.                                                         (                              1                -                                                      cos                    ⁢                                          xe2x80x83                                        ⁢                    φ                                                        cos                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      ⁢                      1                                                                                  )                        xc3x97                          t11              n11                                +                                    (                              1                -                                                      cos                    ⁢                                          xe2x80x83                                        ⁢                    φ                                                        cos                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      ⁢                      3                                                                                  )                        xc3x97                          t12              n12                                      ≦        C2                            (        18        )            
with                     C2        =                              K            xc3x97            B            xc3x97            d            xc3x97            Fno                    -                                    (                              1                -                                                      cos                    ⁢                                          xe2x80x83                                        ⁢                    φ                                                        cos                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      ⁢                      2                                                                                  )                        xc3x97                          t2              n2                                                          (        19        )                                          sin          ⁢                      xe2x80x83                    ⁢                      θ            ⁢            1                          =                              sin            ⁢                          xe2x80x83                        ⁢            φ                    n11                                    (        20        )                                          θ          ⁢          1                =                              sin                          -              1                                ⁡                      (                                          sin                ⁢                                  xe2x80x83                                ⁢                φ                                            n                ⁢                11                                      )                                              (        21        )                                          θ          ⁢          2                =                              sin                          -              1                                ⁡                      (                                          n11                xc3x97                sin                ⁢                                  xe2x80x83                                ⁢                                  θ                  ⁢                  1                                            n2                        )                                              (        22        )                                          θ          ⁢          3                =                              sin                          -              1                                ⁡                      (                                          n2                xc3x97                sin                ⁢                                  xe2x80x83                                ⁢                                  θ                  ⁢                  2                                            n12                        )                                              (        23        )                                0.25        ≦        K        ≤        0.35                            (        24        )                                1        ≦        B        ≦        3                            (        25        )            
t11: thickness of the first birefringent plate
t12: thickness of the second birefringent plate
n11: refractive index of the first birefringent plate
n12: refractive index of the second birefringent plate
t2: thickness of the phase plate
n2: refractive index of phase plate
d: pixel pitch at the imaging device
xcfx86: Angle of incidence at a first birefringent plate of light flux entering corner of the imaging plane of the imaging device from the center of the exit pupil of the taking lens
Fno: F number of the taking lens
The present invention is further provided with a neutralizing circuit for neutralizing electrical charges stored at the first birefringent plate and the second birefringent plate.
In addition, the present invention is provided with a neutralizing circuit for neutralizing at least one of the electrical charges stored at the optical filter, the image forming lens and the imaging device.
The present invention is provided with a photoelectric conversion element for converting an optical image guided to a photosensitive portion of the photoelectric conversion element to an electrical signal, having a cover member covering the photosensitive portion, a transparent electrode formed at a front surface of the cover member and a conductive circuit electrically connected with the transparent electrode and provided to neutralize any electrical charge occurring at the photoelectric conversion element caused by the operation of the electrical system.
Furthermore, the present invention is provided with a photoelectric conversion element for converting an optical image formed by an image forming lens to an electrical signal, an optical member provided in an optical path between the image forming lens and the photoelectric conversion element, a transparent electrode provided, at least, at a surface of an optical member located in the vicinity of the image forming plane of the image forming lens and a conductive member electrically connected with the transparent electrode and provided for neutralizing the electrical charge occurring at the optical member.
The present invention may be further provided with a voltage source that reduces the force with which matter adhering to the photoelectric conversion element by applying a voltage to a conductive member.
The present invention is further provided with a shutter that can be switched between a light blocking state in which a light flux entering the photoelectric conversion element is blocked and an open state in which the light flux is allowed to pass, with the conductive circuit provided to neutralize the electrical charge occurring at the photoelectric conversion element as a result of a shutter operation.
In addition, the present invention may be further provided with a voltage source that reduces the force with which foreign matter adheres to the optical member by applying a voltage to the conductive member.
The present invention is further provided with a control circuit that sustains the open state of the shutter and applies the voltage to the optical member.